This invention relates generally to the mass transport of atoms in a conductor and, in particular, to a process for evaluating the reliability of a thin film interconnector of the type typically used in micro-electronic devices.
As pointed out in U.S. Pat. No. 3,474,530 to Ainslie et al, thin film conductors as typically employed in microcircuits are subjected to a process of electromigration which can under certain conditions lead to early circuit failure. This type of failure generally involves the movement of atoms in the direction of current flow from a first donor region into a second acceptor region. As noted by d'Heurle and Ho in a publication entitled "Thin Films--Interdiffusion and Reactions" published by Wiley-interscience, New York 243-303 (1978), electromigration failure occurs in two separate stages. During the first stage of failure, herein referred to as the electromigration damage (EMD) stage, atoms move out of the donor region under relatively well defined conditions leaving behind voids in the material. The transported atoms are deposited in the acceptor region thereby creating hillocks. The second stage of electromigration failure, which is herein referred to as the catastrophic failure process or (CFP) stage, is characterized by complex temperature and current density variations that lead to a rapid and complete failure of the device.
It is important to note that the two stages of electromigration failure occur in sequence with (EMD) being first in time. The damage that takes place in these early stages of the process proceed under well defined conditions of temperature, temperature distribution, and current density. These conditions remain relatively constant during (EMD) and to a great extent controls the failure process over most of the conductor's life. The second, more dramatic, stage of the failure process, while still an electromigration event, is not characterized by the initial conditions of temperature and current density previously experienced by the conductor but rather by local current densities and temperatures that develop in the now highly stressed donor region. Microscopic defects produced in the conductor by complex temperature and current density variations increase flux divergences in the previously damaged regions thus bringing on rapid, catastrophic and total failure. Although the second stage of failure is a consequence of the first, it nevertheless occurs with rapid kinetics and under less well-defined conditions than those experienced during the earlier stages.
It is important to note that (EMD) occurs over a major portion of the conductor life while (CFP) takes place during a relatively short period at the end of this life span. Accordingly, the physical changes in the conductor which controls the overall failure process typically requires an extremely long period of time to produce catastrophic failure. Heretofore, the kinetics of an electromigration process have been determined by life test experiments known as Mean Time to Failure (MTF) tests. In this type of testing a specimen is generally electrically stressed under isothermal conditions. The kinetics of electromigration are then determined as weighted averages which are taken over a relatively long period of time beginning with the initial stressing of the specimen and ending with failure. As can be seen, these weighted averages are characteristic of both (EMD) and (CFP). The results obtained are therefore of a generally questionable nature in light of the fact that (CFP) depends upon the initial state of the specimen and the specific damage produced during the first stage (EMD) of the process. Testing of many samples is needed to determine (MTF). These tests clearly show that the life span of the specimens can vary by as much as a factor of four. Furthermore, using the test results to determine kinetic parameters relating to the process is also questionable because, as noted, local temperatures and current densities change dramatically during the latter stages of failure and, as a consequence, the time-to-failure values are sometimes more likely to reflect variations in (CFP) rather than (EMD).
Electrical resistance or resistivity measurements have also been used to study the kinetics of the electromigration process. However, like (MTF) measurements, these isothermal resistivity measurements require testing of many samples over long periods of time to determine the kinetic parameters of electromigration. A more thorough treatment of this type of testing is given by Hummel et al, in the Journal of Physics and Chemistry of Solids, Pergamon Press, 1967, Vol. 37, at pp. 73-80, (printed in Great Britain).